Get yourself one or two one handed sword depend of their quality
16 and lower expect to have 2 or even 3.
16 - 40 get two of them (Royale sword is a good one to get if you can
)
40+ Is there any one handed sword that strong ?
First try to sneak close the mobile Guardian as much as you can.
Once you are at it's legs stop it with "Cinetis +" (that tab power that stop in time items and monster.)(Be careful it does not stay for more than 1 or 2 secondes on this monster.)
Then cut it legs and much as you can. (circular attack help a lot that why one handed sword is suggested!)
If you manage to cut all of it's legs it will become a static guardian with droplist of mobile guardians ( aka it still drop that core and giants core
)
then repeat it as much as you can.
Stop in time
Smash like a crazy.